There are many instances in a manufacturing setting where two aluminum alloy workpieces need to be joined along a shared interface. The automotive industry, for example, often chooses to laser weld aluminum alloy panels when constructing certain vehicle component parts. These vehicle component parts can include the roof, decklid, and trunk, to name but a few. And when laser welding is necessitated at what will become a Class A surface of the component part, which is typically a styled and non-planar visible surface on the vehicle exterior, the aesthetic appearance of the weld joint takes on added significance. The laser weld joint, in particular, must meet certain acceptable appearance standards so that, when the vehicle component part is painted, the show surface of the part is not rendered aesthetically unappealing.
Laser welding is a metal joining process in which a laser beam provides the energy needed to effectuate welding. In practice, a laser optic head focuses and directs the laser beam at a weld seam established between the two workpieces while a robot arm moves the laser optic head to translate the laser beam along the weld seam. When laser welding together two aluminum alloy workpieces, a filler wire is typically tracked along the weld seam in coordination with the movement of the laser beam so that a working end of the filler wire is impinged by the laser beam in the presence of a shielding gas. The filler wire absorbs the energy of the laser beam and melts in the protective shielding gas environment to deposit a molten filler material along the weld seam. Portions of the aluminum alloy workpieces adjacent to and along the weld seam are also typically melted at the same time as the filler wire. The molten filler material ultimately interacts with the molten aluminum alloy present at the weld seam and, upon further advancement of the laser beam and filler wire, cools and solidifies. A laser weld joint is thus produced in the wake of the coordinated movement of the laser beam and the filler wire along the weld seam.
A high energy density laser beam is typically needed to concentrate heat within the aluminum alloy workpieces during laser welding on account of the relatively high thermal conductivity of aluminum alloys. Such a strong laser beam tends to produce a keyhole—which is a column of metal vapor and plasma—at the weld seam in the immediate surrounding vicinity of the laser beam. The keyhole penetrates into the aluminum alloy workpieces and initiates lateral melting of the workpieces to establish a surrounding molten weld pool that follows the path of the keyhole. While the produced keyhole allows for deeper melting of the aluminum alloy workpieces along the weld seam, it has a tendency to detract from the smoothness and overall visible appearance of the weld joint. The instability and turbulence of the keyhole, in particular, introduces porosity into the molten weld pool when the keyhole collapses and causes the weld joint to have a wavy and uneven surface appearance. Because of this, the weld joint usually has to be buffed or brushed or sanded before the laser welded part can be painted, which adds time and complexity to the overall manufacturing process.